The invention relates to a high frequency apparatus for heat treatment of a biological tissue.
High frequency apparatus is already known which effects the excision or coagulation of a biological tissue or lymph to stop the hemorrhage by passing a high frequency current having a frequency on the order of several hundreds kHZ to several MHz through a biological tissue such as the stomach, intestines, liver or the like so that Joule' s heat may be produced by the electrical resistance which the tissue presents and the current flow therethrough. In a conventional high frequency apparatus of this kind, the load characteristic defining the relationship between a load current which passes through the tissue to be treated and the voltage applied across electrodes which are directly applied against the tissue is chosen to exhibit a constant voltage characteristic, with a no-load voltage in excess of 1000 volts peak-to-peak. When such high frequency apparatus is used in combination with so-called bipolar electrodes for the purpose of coagulation, an inconvenience is caused in that during the energization, a spark discharge may be produced between the tissue and the electrode when the electrical resistance of the tissue engaged by the electrode increases as a result of a change in its composition. The discharge will be initiated at that electrode having a greater contact resistance with the tissue than the other, and in the region of discharge, the potential gradient will be substantially increased as compared with that in the region of the other electrode, whereby the generation of heat will be concentrated only in the region of the first mentioned electrode, resulting in a coagulation of only that portion of the tissue which is located adjacent thereto. In this manner, it has been difficult heretofore to achieve a uniform coagulation of the tissue located across the pair of electrodes. Such failure is liable to occur when the electrodes are spaced apart over an increased distance in order to achieve a coagulation over an extensive area. As a result, when the conventional high frequency apparatus is used in combination with bipolar electrodes, by which term is meant an electrode arrangement which is adapted to hold a tissue portion to be treated between a pair of electrodes, the spacing between the electrodes had to be reduced. Where a coagulation of the tissue over an extensive area is desired, a so-called monopolar electrode is used. The monopolar electrode is applied against a tissue portion to be treated, while the other electrode is held stationary against a distinct portion of the tissue. However, with a monopolar electrode, the coagulating action is not limited to the surface portion, but may proceed into the internal tissue portion, thereby disadvantageously coagulating a normal tissue.